This invention relates to bioimplantable polymeric articles and more particularly to methods of improving the wear resistance and oxidation resistance of such articles.
Advances in biomedical engineering have resulted in numerous polymeric articles which are able to be implanted within the body. Polymeric components are widely used in orthopedic surgery, for example, to form articulation surfaces within artificial joints. Ultrahigh molecular weight polyethylene (UHMWPE) is an example of a polymer that is commonly used to form components of artificial joints.
Among the properties required of bioimplantable polymeric components, particularly those used in artificial joints, are low friction, biocompatibility, and good mechanical properties, including excellent resistance to wear. Such components must also be sterile before implantation within a patient.
Some polymers and medical devices may be adversely affected by heat sterilization, thus such a technique is not widely used. Ethylene oxide sterilization is another technique for sterilizing medical devices, but ethylene oxide can pose health and enviromnental risks that render this method less desirable. As a result, a preferred method of sterilizing many medical devices, including polymeric components, is by exposure to forms of ionizing radiation such as gamma rays, x-rays, or electron beam radiation.
Presently, sterilization by gamma radiation is a preferred method for sterilizing many medical devices, including bioimplantable polymeric components. One potential effect of gamma radiation sterilization is that the gamma rays can initiate chemical reactions within the polymer that can affect the structure, morphology and some mechanical properties of the polymer. During gamma irradiation a variety of chemical species, such as ions, excited molecules, double bonds, oxidation products and free radicals are created within the polymer. Free radicals are believed to be a species generated during gamma radiation that may contribute most to changes in the properties of irradiated polymers.
Once the radicals are formed within a polymer, these species may participate in at least four types of major reactions. The free radicals can undergo a recombination reaction by reacting with hydrogen to eliminate the free radical, by reacting with carbon molecules to create side chains, or both. Free radicals can also undergo a chain scission reaction that results in a decrease in the molecular weight of the polymer, and an increase in the density and crystallinity of the polymer, thus causing some mechanical properties of the polymer to degrade. A crosslinking reaction is another reaction in which the free radicals can participate. Finally, the free radicals may remain within a polymeric material without reacting initially, thus remaining available to react over time as conditions dictate.
The presence of oxygen in polymeric materials and their surrounding environment can contribute to an oxidation reaction in which free radicals and dissolved oxygen react to produce a compound with a carbonyl functional group, resulting in chain scission and the creation of new free radicals. Oxidation can decrease the molecular weight of a polymer (due to chain scission) and contribute to the degradation of its mechanical properties.
Sterilization of polymer components by gamma radiation in air is believed to decrease the wear resistance of polymers due, in part, to oxidation effects. Wear resistance is a key mechanical property for polymeric components that are used in joint prostheses. As a result, a current practice is to sterilize polymeric components in an environment of an inert gas (e.g., argon, helium, nitrogen)to minimize oxidation effects. See, Kurth, M. et al., "Effects of Radiation Sterilization on UHMW-Polyethylene" Antec 87, pp. 1193-1197 (1987); Streicher, R. K., Radiol. Phys. Chem., Vol. 31, Nos. 4-6, pp. 693-698 (1988); Streicher, R. M., "Improving UHMWPE by Ionizing Radiation Crosslinking During Sterilization", 17th Annual Meeting of the Society for BioMaterials, p. 181 (1991). Others have used vacuum techniques to help purge an environment of oxygen before conducting gamma radiation sterilization. See, Yong Zhao, et al., J. Appl. Polymer Sci., Vol. 50, pp. 1797-1801 (1993).
Wear resistance is a property of great importance to artificial joint components. Natural friction within a replaced, artificial joint can cause minute particles of debris (e.g., particles from a polymeric component)to become dislodged and to migrate within the joint. This phenomenon of wear debris within artificial joints is a serious problem that can inhibit the proper mechanical functioning of the joint. Wear debris can also lead to osteolysis and bone deterioration. If osteolysis develops around an artificial joint it is usually corrected by surgical removal of the diseased tissue and revision of the artificial joint.
Because excellent wear resistance is a property of such importance for polymeric components used to form artificial joints, it would be advantageous to be able to provide sterilized polymer components that have improved wear resistance.
It is thus an object of the invention to provide methods for increasing the wear resistance of bioimplantable polymeric components. It is also an object to provide sterilization techniques for medical grade implantable polymer components that preserve important properties of the components. A further object is to provide bioimplantable polymeric components that have improved wear resistance and that are less prone to the effects of oxidation. These and other objects will be apparent to one of ordinary skill in the art upon reading the description that follows.